1. Field of the Invention
The present invention relates quite generally to the field of LED modules, i.e. modules in which an LED chip (“LED die”) is applied to a mount, generally a platform. It is known in the case of such LED modules, inter alia for mechanically protecting bonding wires, for example, or else for influencing optical properties of the light emitted by the LED chip, to then apply (“dispense”) a light-transmissive layer over the LED chip by means of known techniques (for example stenciling technique).
2. Related Technology
There are various technologies for applying the LED chip to the platform. Using the so-called chip-on-board (COB) technique, the light-emitting LED chip is normally placed directly onto a printed circuit board and then encapsulated with the transparent dispensed material. Using surface-mount technology (SMT), the chip is generally surrounded by a reflective material in order to reduce the proportion of light which could otherwise be lost as a result of scattering effects.
The entirety of the material surrounding the LED chip (reflector walls, platform, etc.) will be referred to below as a “package”. In addition to improving the efficiency by virtue of pre-aligning the emitted light by means of reflective surfaces (ceramic, metal, etc.), the object of the “package” is to ensure in particular the electrical supply to the LED chip (for example by means of “vias” through the package or bonding wires) and to provide effective heat dissipation from the LED chip to the surrounding environment.
Silicone and epoxy resin, for example, are known as transparent dispensed material. The transparent dispensed material can contain, if appropriate, wavelength-converting substances (referred to below as phosphors), scattering particles for improved mixing of the converted spectrum with the spectrum originally emitted by the LED chip and additives for adapting rheological parameters such as viscosity, storage modulus and loss modulus.
The uncured mixture of the dispensed material with optionally taken phosphors, viscosity additives etc. is also referred to below as “paste”.
In the COB sector, high-viscosity pastes are generally used for dispensing, i.e. pastes with a viscosity of more than 50 Pas, and a storage modulus of more than 100 and preferably 500-1000 in order to ensure an approximally hemispherical surface form and mechanical stability and dimensional stability of the dispensed layer.
In the case of very compact packages in which possibly a plurality of cavities for accommodating LED chips are positioned tightly packed next to one another on a silicon wafer, dispensing such high-viscosity pastes can, however, present problems owing to possible air inclusions. The risk of air inclusions can be reduced by lowering the viscosity and also by increasing the shear force during the dispensing process. However, in this case, the risk of the silicone flowing together between two or more adjacent cavities is increased.
It is known from EP1786045A2 to apply a dispensed layer over an LED chip applied in a depression in a platform by means of a dispensing process.
It is known from US 2006/0199293 A1 to dispense an epoxy resin with a viscosity of 2000 to 3000 cP (from 2 to 3 Pas) over an LED chip.